Electronic signal amplifier circuits are used for a variety of purposes. For example, they can operate as an audio frequency preamplifier that boosts the magnitude of signals obtained from an audio source, or from an instrumentation source having an audible frequency range. Some audio frequency power amplifiers can be directly connected to an input source and/or an output without use of capacitors or coupling transformers. Audio signal amplifiers can also be used as power amplifiers that boost the magnitude of audio frequency signals obtained from a preamplifier.
Preamplifiers and power amplifiers for audio frequency signals have been used for these and various other functions. For example, U.S. Pat. No. 4,719,431 granted to Karsten in January 1988, and U.S. Pat. No. 6,242,977 granted to Karsten in June 2001, both disclose power amplifiers. Some preamplifiers have been shown to be advantageous in being capable of transferring output signals therefrom over a balanced line interconnection arrangement directly connected to its output. For example, the so called “Circlotron” circuit is such a high performance preamplifier.